Over the past decade, the use of a wide variety of pigmented thermoformed shaped articles including molded and extruded articles, has grown apace despite the known tendency of such articles to change color long before they lose their integrity. A large proportion of such articles are formed from stabilized polyolefins, and other normally solid predominantly ethylene- and propylene-containing copolymers (together referred to hereafter as "PO" for brevity), and are used for a host of different applications. Providing such articles with adequate stabilization against photooxidative degradation is a difficult and complicated problem (see "The Photooxidative Degradation of Polypropylene. Part II. Photostabilization Mechanisms" by Carlsson D. J. and Wiles, D. M. J. Macromol. Sci.--Rev. Macromol. Chem., C14(2), 155-192 (1976)). Whether molded or extruded, whether in thick sections or in thin (such as fibers and sheets) loss of color is a particularly acute problem for PO pigmented with azo and disazo pigments (together referred to herein as "azo pigments") which in most other respects are ideal pigments for polyethylene (PE) and polypropylene (PP). Typical of such azo pigments is Red 144 (common name). It is used in huge (tons) quantities to pigment PO.
Retention of colors red, yellow, and orange, and shades of these colors, derived from azo pigments in articles exposed to sunlight over their useful life, is of great practical value in molded PO household goods, and in clothing made from woven or non-woven fabrics of the pigmented PO fibers or sheet. But Red 144, Orange 34 and Yellow 93 are known prodegradants; and, PO pigmented with these pigments, not only degrades the PO but it loses its pigmentation due to chemical degradation of the pigment even before the PO itself is degraded past the point where the articles serve their intended use.
To combat the problem of color fading, a better solution is constantly being sought to decelerate color loss which is at least as important as stabilization of the mass of the PO. With particular respect to Red 144-pigmented PP articles which are in high demand, the use of Red 144 both hastens their degradation when exposed to sunlight, and degrades their physical properties over time. The combined effects of such degradation result in a two-pronged attack on the longevity of the articles in normal use, thus vitiating their marketability.
In particular, fabrics made with Red 144-pigmented PP fibers and stabilized with hindered amine light stabilizers (HALS), are especially popular in automobiles, boats, outdoor clothing and other such uses where the fibers degrade at such an unacceptably high rate upon exposure to sunlight, that red articles are soon transformed into non-uniformly colored articles sporting a wide spectrum of unwanted shades of pink and orange. The obvious way to cope with this color degradation problem is to use far more pigment than is required to provide the desired color, so that upon suffering the expected color degradation, the coloration of the remaining non-degraded pigment will maintain acceptable, if not the original, color. Except that `loading up` the HALS-stabilized fibers with more Red 144 or other azo pigment generates a high proclivity towards reaction of pigment with the commonly used HALS, and other additives such as antioxidants ("AO"s) and antiozonants, used to provide melt-stability to the PP. Further, increasing the concentration of pigment above about 1 phr may produce "blooming" of the pigment long before degradation of either the pigment or the fiber.
This invention particularly relates to the stabilization of shaped articles of PO, specifically of PE and PP articles colored with azo pigments which provide colors across the entire visible spectrum; more particularly, it relates to those pigments which provide either a red, orange, brown, or yellow color, and various shades thereof.
It is known that several stabilizers, particularly the HALS, by themselves, provide excellent stabilization of PO to heat, light and ultraviolet radiation, and, some hindered phenol stabilizers are antioxidants which provide both excellent thermoxidative stabilization, and light stabilization of PO, but such stabilization does not extend to that of color in azo-pigmented PO. For example, a hindered piperidyl compound such as is commercially available as Chimassorb 944, has been combined with a benzoate type stabilizer such as 3,5-disubstituted-4-hydroxybenzoate (generically referred to as "3,5-DHBZ"), and a phosphite commercially available as Ultranox (see Japanese publication JP-230401 (1987) to Sumitomo Chem Ind KK). But there is no indication that such a combination might have been notably effective to stabilize any pigment or dye.
It is also known that numerous pigments for PO, by themselves, provide a significant level of stabilization to PO, but there are many which have no noticeable effect on stability, and still others which accelerate degradation, that is, are prodegradants. To date, the only reliable method of determining to which group a pigment belongs, is by actually testing it in a particular substrate of interest. It is known, for example, a pigment which is a stabilizer in PP may be a prodegradant in a polyacetal.
Azo pigments are known to have good color stability. They also provide some measure of light stability by virtue of their ability to block the path of radiation, thus shielding the azo-pigmented polymer. Such small measure of light stabilization is observed in PP at about 0.4 phr. However, in combination with a stabilizing amount of a known HALS primary stabilizer, which functions as a "radical trap" stabilizer, stabilization provided by 0.6 phr (for which data is provided herein) and as much as 1 phr, of an azo pigment is not substantial.
By a "primary" stabilizer, I refer to one which provides either long term thermoxidative stability during conditions to be encountered during use, or, UV light stability in bright, direct sunlight. Melt extrusion stability, especially for fibers of PO, is provided by a secondary stabilizer. Though the present invention does not require the use of any secondary stabilizer, in those instances, for example in fiber-spinning, where the melt is extruded at about 270.degree. C. (for PP fibers) more than once to obtain better pigment distribution, the PO may contain a small amount, no more than 0.1 phr of a melt (or "process") stabilizer.
Typically, several additives are combined in PP before it is thermoformed, whether spun into fiber, injection molded, blow molded, extruded, etc., each additive being specifically designed to provide a different zone of stabilization, the main zones being (a) melt extrusion stability, (b) long term thermal stability during conditions expected to be encountered during use, (c) uv light stability in bright direct sunlight, and by no means of least importance, (d) stable tinctorial strength to maintain the desired color. Combining several additives known to be effective for each purpose, in PP articles is not likely to produce the desired results because of objectionable side effects due to interaction between the additives.
For example, thiodipropionate compounds such as dilauryl (DLTDP) and distearyl (DSTDP) help control melt-stability despite an odor problem, and certain phosphites control melt flow while depressing the tendency of PP fibers to yellow because the fibers usually contain a hindered phenol AO. The hindered phenol AO increases long term stability but accelerates yellowing. It is known that a hindered phenol AO and a thiodiproionate are most effective when used together. Certain HALS provide not only excellent uv stability but also such good long term thermal stability that the PP fibers will outlast some of the pigments used to color them. Yet a HALS is typically combined with a hindered phenol and a phosphite.
Conventional wisdom dictates that if fibers stabilized with one or more stabilizers and a particular pigment meet the expectations of stability in the marketplace, then molded and extruded articles other than fibers will also be satisfactorily stabilized. The opposite is not true. Therefore, pigments are selected with an eye to their effect upon the processing of PP fibers, the stability requirements of the end product, the pigment's interaction with the other additives to be used, the color requirements, and the cost of producing the pigmented PP fibers. The intense thrust towards using inexpensive PP fibers in the automobile industry where the color red is in high demand, decreed that, despite its high cost, Red 144 be used, because of its intense tinctorial strength and color stability; and, that Red 144 be combined with a compatible uv stabilizer. I found that one of the most damaging factors in the stability of Red 144-pigmented fibers was their interaction with the HALS used.
In my U.S. patent application Ser. No. 07/352,519 filed May 16, 1989, I claimed the discovery that N-(substituted)-.alpha.-(3,5-dialkyl-4-hydroxyphenyl)-.alpha.,.alpha.-disu bstituted acetamides ("3,5-DHPZNA") in which one of the substituents on the N atom is a 2-piperazinone group, essentially by itself, except for a melt-stabilizing quantity of a secondary stabilizer, provided excellent discoloration resistance to Red 144-pigmented fibers of PP. It was known that the 3,5-DHPZNA by itself was a good uv stabilizer, as disclosed in U.S. Pat. No. 4,780,495 to John Lai. Surprisingly, when plaques (because they are most conveniently prepared) of Red 144-pigmented PP were tested, they were found to deteriorate rapidly. The Red 144-pigmented fibers were found not to.
The commercial use of red PP fibers requires that the color stability of the PP fiber be such that it equals the useful life of a fabric or other article containing the fiber, which article is exposed to heat and light. Because the stabilizers used generally affect color though they are not regarded as colorants, and, pigments affect thermal and uv light stability even if not known to have such activity, one cannot estimate what the net effect of the interactions might be. (see "Influence of Pigments on the Light Stability of Polymers: A Critical Review" by Peter P. Klemchuk, Polymer Photochemistry 3 pg 1-27, 1983).
I continued my tests with numerous combinations of stabilizers in Red 144- and other azo-pigmented fibers, screening the samples to determine whether an unacceptable level of color loss was obtained before the fibers disintegrated. It was unimportant whether the combinations were of a primary with a secondary stabilizer, or, of co-primary stabilizers. The effectiveness of each combination was measured by the degree of degradation of the pigmented fibers both by visual observation, and by "scratch testing" (described herebelow) the surface of exposed fibers.
Fiber degradation is a phenomenon which is easily visible to the naked eye upon inspection of a degrading pigmented yarn exposed either in a Weather-O-Meter in the presence of moisture, or, to bright sun (tests are conducted in the Florida sun) under ambient conditions of humidity. Unstabilized Red-144 pigmented PP fibers exposed to the Florida sun show no fading because the pigmented fibers degrade far more rapidly than the pigment, which results in continual sloughing off of layers of fiber, exposing bright undegraded pigment. Degradation of stabilized fiber is characterized by (i) a fuzzy, peach-skin-like appearance of the surface of the fabric (made with the pigmented fibers), and (ii) the problem of fading color.
For the simple reason that a large volume of azo-pigmented PP goods are either extruded or molded, one way or the other, there was an urgent need to find an effective stabilizer system which would provide such articles, as well as fibers, with adequate longevity under light-degrading conditions. To this end I searched for the appropriate HALS and for a compatible and effective co-stabilizer or "synergist" which might, in combination, provide the desired stabilization. Since there was no indication whether such a synergist should be, or might likely be either an AO or a uv-absorber, the search had to consider both.
As one would expect, some pigments enhance heat and light stability of PP fibers stabilized with a particular AO or uv-absorber. Other pigments have the opposite effect. Until tested, one cannot predict with reasonable certainty, what the effect will be. For example, with a nickel-containing stabilizer, Red 101 (iron oxide) is a prodegradant. With the more effective HALS, both Yellow 93 and Red 144 are prodegradants. The effect of these pigments in stabilized PP fibers could not have been predicted by their behavior in unstabilized pigmented fibers, or by their behavior with a different stabilizer.
With a nickel-containing stabilizer, Red 144 (unlike Red 101) is a stabilizer (not a prodegradant), but Red 144 is a prodegradant with Tinuvin 770. Yellow 93, a stabilizer when no other stabilizer is present, is neutral with nickel stabilization but is a prodegradant with Tinuvin 770 (see "Stabilization of Polypropylene Fibers" by Marvin Wishman of Phillips Fibers Corporation). Specifically with respect to red PP fibers, the problem was to find a combination of stabilizers which circumvented the proclivity of Red 144 to degrade the PP fibers and plaques when the pigment is combined with a conventional AO and uv light stabilizer. Because Red 144 was a prodegradant, it seemed desirable to use only as much of it as would provide the desired tinctorial effect for the required period of time, namely the useful life of the stabilized fiber.
The effect of a large number of pigments on the stability of PP fibers stabilized with Tinuvin 770 has been reported by Steinlein and Saar (see "Influence of Pigments on the Degradation of Polypropylene Fibers on Exposure to Light and Weather", paper presented at the 19th International Manmade Fiber Conference, Sep. 1980 in Austria).
In the same vein, like other workers before me, I tested a large number of combinations of primary stabilizers with Red 144, both in PP fibers and in relatively thick (0.5 mm) PP film.
The chemical peculiarity about a PIP-T is that it contains at least one triazine ring, and all substitutable positions on each triazine ring are polysubstituted piperazin-2-ones. Chimassorb 944 is a HALS of comparable size to that of a PIP-T, and like PIP-T is a hybrid molecule containing a hindered amine (piperidine) and a triazine ring. Chimassorb 944 is an oligomer in which the repeating unit combines a hexamethylene diamine having polysubstituted piperidyl substituents on the N atoms, the substituted diamine unit being connected to a triazine ring in which one of the other substituents is a branched chain alkylamine, and the last substituent is also a hexamethylene diamine unit.
Mainly because Chimassorb 944 contains pendant piperidyl rings rather than piperazinone rings, but perhaps also because of the relatively elongated molecule, we found that Chimassorb 944, used in combination with a 3,5-DHBZ, is not as effective with azo pigmented PP as the combination of the 3,5-DHBZ with a PIP-T. The unexpected and particularly noteworthy boost of color-stability derived from a 3,5-DHBZ, is thought to be due to the electron-withdrawing effect of the para-position of the ester substituent, but the highly surprising effect when the ester group is aryl, for example, 2,4-di-t-butyl is thought to be attributable to the photo Fries rearrangement (when the 3,5-DHBZ is exposed to actinic radiation) which rearrangement can occur easily only with the aryl ester substituent. Other esters, particularly those derived from alicyclic or long chain aliphatic alcohols are comparably effective, though evidently not for the same reason.
Prior to the publication of some of the studies which set forth a framework within which the foregoing factors are to be considered, a manufacturer of the 3,5-DHBZ supplied a sales flyer in which its UV-Chek AM-340 stabilizer was stated to be "cost effective or synergistic with hindered amine light stabilizers, as well as, other stabilizer types, e.g., nickel organics, benzophenones, benzotriazoles, etc. in various polymers." The flyer further stated "In polymers, particularly polyolefins, UV-Chek AM-340 is a highly effective ultraviolet light stabilizer. It is especially effective in extruded and blown films, fine fibers, and molded articles. AM-340 is a white crystalline powder, and has no effect on the initial color of the polymer it is added to, and no effect on changing the shade of pigmented formulations. AM-340 is particularly useful in stabilizing pigmented polymers, especially in cases where the pigment itself contributes to the degradation of the polymer."
To the extent that such all-encompassing benefits of using AM-340 were not sales-oriented, they were based on data obtained with oriented PP film in 1.times.100 mil and 2.times.100 mil samples, and on 20 mil HDPE and 4 mil LLDPE plaques pigmented with 0.5% titanium dioxide. No observed color changes are provided, nor is there any indication that they were measured and found to show no change.
The PIP-T (see U.S. Pat. No. 4,480,092 to Lai and Son), which was discovered more than a decade after publication of the flyer, is acknowledged to be an excellent uv stabilizer and, because I found the combination of PIP-Ts with 3,5-DHBZ was effective with phthalocyanine-pigmented PO, I tested the same combination with the azo pigments.